The Trump-Kim Summit

A diplomatic breakthrough is easy when you offer the other side what it wants. And Donald Trump on Thursday gave North Korea something it has long craved: a summit with a sitting U.S. President. Perhaps this will be the start of a stunning nuclear disarmament, but it could also end up in a strategic defeat for the United States and world order.

Mr. Trump doesn’t do normal diplomacy, and this leap to a face-to-face meeting had his impulsive trademarks: spur of the moment in response to a Kim Jong Un offer relayed through South…